Outline

Background: Filaminopathy and desminopathy are subtypes of myofibrillar myopathy (MFM) caused by mutations inFLNC, the gene encoding filamin C, andDES, the gene encoding desmin. A histopathological hallmark of MFM is a massive protein aggregation within skeletal muscle fibers. The aim of our study was to elucidate the composition of aggregates from patients with differentFLNCandDESmutations by using a label-free mass spectrometry approach.

Methods: Aggregates and control tissue from muscle biopsies of 6 MFM patients with 3 differentFLNCmutations and 4 MFM patients with 4 differentDESmutations were collected by laser microdissection and analyzed by a combination of mass-spectrometry and spectral index calculation. Proteins with a ratio >1.8 (aggregates compared to intraindividual controls) were accepted as accumulated in aggregates. Results of selected proteins were validated by immunofluorescence studies. Mass spectrometric data were searched against an extended human protein database to detect the mutations at the protein level.

Results: In filaminopathy samples, 392 different proteins were identified and 69 of them showed an accumulation in aggregates. The composition of aggregates was highly similar in patients with differentFLNCmutations with high abundance of filamin C. Aggregate compositions in desminopathy were more heterogeneous but desmin was the most abundant protein in all cases except of one. 317 different proteins were identified and 98 of them showed an accumulation in aggregates. Immunolocalization findings were consistent with proteomic data. 3 out of 4 desmin mutations were identified at the protein level.

Conclusion: Our proteomic approach detected many novel components of protein aggregates within skeletal muscle fibers of filaminopathy and desminopathy patients and enabled the identification of different desmin mutations at the protein level. The highly similar aggregate composition in filaminopathy indicates common pathomechanisms. In contrast, proteomic findings in desminopathy suggest differences in pathogenesis depending on individual mutation, consistent with data of previous functional studies.